2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/143959The Beresinskii-Kosterlitz-Thouless (BKT) physics of vortices in two-dimensional superconductors at finite magnetic field is investigated by means of a field-theoretical approach based on the sine-Gordon model. This description leads to a straightforward definition of the field-induced magnetization and shows that the persistence of non-linear effects at low fields above the transition is a typical signature of the fast divergence of the correlation length within the BKT theory.4 pages, 2 figures, to appear on Physical Review LettersSuperconductivityStatistical Mechanicstext